(i) Field of the Invention
This invention relates to abradable seals and, more particularly, relates to high-temperature abradable seal compositions for use in gas turbine engines, turbochargers, compressors, steam turbines and the like.
(ii) Description of the Related Art
Basic requirements for abradable seals in the compressor section of gas turbine engines include good abradability, spall resistance, and erosion resistance. Abradable seals are also required to exhibit low gas permeability, a smooth surface, good aging properties and long-term oxidation resistance at high temperatures. In the case of abradability, the seal is a sacrificial element, it being desirable to minimize blade wear. Additionally, low gas permeability is required in order to minimize gas flow through the seal itself. It has been shown that low permeable seals with a smooth surface finish improve overall compressor efficiency by about one percent as compared to conventional permeable seals. In addition, low permeability of the seal prevents entrapment of fine particles, e.g. dust or grit, which can act as an abrasive against the blade tips, thus wearing them unevenly. Smooth surface finishes in the gas path improve overall airflow, also contributing to efficiency. Finally, long-term oxidation resistance is required due to increases in compressor operating temperature up to 815° C.
There are several air seals used in a compressor section of a gas or aircraft engine. Historically the oldest is feltmetal that comprises a plurality of metal fibres. The feltmetal is described for example in U.S. Pat. No. 4,257,735. The most important disadvantages of this seal are that it has to be brazed to the substrate material and that it is highly porous.
Typical jet engine compressor air seals include a metal matrix of aluminum-silicon with embedded polymer particles or hexagonal boron nitride powder particles as described in U.S. Pat. No. 3,723,165 and U.S. Pat. No. 5,506,055, respectively. The disadvantage of these systems is their limited temperature capability at 315° C. for the system with polymer and 480° C. for the system with hexagonal boron nitride. In the former case, the temperature capability is governed by the polymer and in the latter case it is governed by the aluminum silicon alloy.
Abradable materials used at high temperatures in the compressor section of turbine engines are usually NiCrAl/Bentonite coatings described in U.S. Pat. Nos. 4,374,173 and 4,291,089 by Adamovic. However, NiCrAl/Bentonite seals do not rub well against Ti alloy blades. These coatings perform well against Ni alloy and steel blades but, when Ti alloy blades are used, the blade tips overheat and are subject to wear. Sometimes, glazing of the coating is observed.
Another known abradable seal is that prepared by the techniques of Rangaswamy et al., described in U.S. Pat. No. 5,434,210. A composite powder for thermal spraying of abradable coatings is disclosed in which the composite powder contains three components. One component is any of a number of metal or ceramic matrix materials, another component is a solid lubricant, and the third component is a polymer. Typical as-sprayed coatings comprise a Co alloy matrix with dispersed particles of hexagonal boron nitride and polymer. The polymer is subsequently burned out and the final very porous structure contains only hexagonal boron nitride particles dispersed throughout the Co-based matrix. The coatings prepared from this material have acceptable abradability but low erosion resistance. The erosion resistance is required in order to maintain uniform clearances throughout the life of the engine or engine performance characteristics are adversely affected. Conventional commercial turbine engines have exhibited a two percent increase in airflow around blade tips as a result of seal erosion after approximately 3,000 flights. Much of this may be attributed to erosion of the abradable seal and blade airfoil tip, and to rub interactions between the blade tips and the seal. In military engine applications, where gas path velocities are relatively high, erosion resistance is of paramount importance.
Moreover, high permeability due to open porosity of conventional seals enables back leakage of gas, which decreases engine efficiency.
We have found that the use of Ni alloy-clad hexagonal boron nitride powder applied by flame or plasma spraying provided an abradable seal which exhibited poor combination of erosion resistance and abradability. When erosion resistance was acceptable, the abradability was poor. When abradability was satisfactory, the erosion resistance was poor. Ni alloy-clad boron nitride, accordingly, is not suitable for use as an abradable seal.
It is accordingly a principal object of the present invention to provide a novel thermal spray composition and its method of application for producing an abradable seal.
A further object of the invention is the provision of an abradable seal, for use in gas turbine engines having good abradability, spall resistance and erosion resistance, particularly when used in conjunction with titanium-alloy blades.
It is another object of the present invention to provide an abradable seal having a smooth surface, low permeability, good thermal conductivity, low interparticle cohesive strength and long-term oxidation and glazing resistance resulting in favourable long-term aging characteristics.